1. Field
The present invention relates to a lithographic apparatus, a method for shielding an element of a lithographic apparatus for air flows and pressure waves and a device manufacturing method.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In the lithographic process of a lithographic apparatus, it is desirable that at least the patterning device, the projection system and the substrate stage be properly aligned with respect to one another so that the pattern, which is provided by the patterning device in the radiation beam, is properly projected on a target portion of the substrate without overlay errors, imaging errors or focus errors. In particular, in scanners, in which the patterning device support (e.g. reticle stage) and the substrate table (e.g. substrate stage) are movable to position a particular part of the pattern with respect to a particular part of the substrate, high accuracy positioning is desirable. For these movements, positioning systems are provided which control the position of the reticle stage and substrate stage with high accuracy.
With continuously increasing demands on the accuracy of imaging, for instance overlay and focus on the one hand and throughput on the other, proper alignment of the reticle stage, projection system and substrate table becomes increasingly critical. In order to increase the throughput of the lithographic apparatus, it is desirable to increase the speed and acceleration with which the reticle stage and substrate stage are moved and aligned with respect to each other and the projection system.
However, moving the reticle stage or the substrate stage results in air flows and pressure waves which propagate through the space in which these stages but also the projection system are present. Also, the actuation forces of the stage may cause vibrations of parts of the stage resulting in air flows and/or pressure waves, such as acoustic signals or air flows through the working space. These air flows and/or pressure waves may excite the projection system, or at least parts of the projection system such as the lenses, or the frame on which the projection system is mounted. The air flows and/or pressure waves may also excite other parts of the lithographic apparatus being relevant for the alignment of the reticle stage projection system and wafer stage such as sensor or sensor target object of a stage position measurement system. The excitation of the projection system, or the other parts, may cause imaging errors such as errors in overlay and/or focus.
Also, other moving or vibrating parts in the lithographic apparatus may provide air flows and/or pressure waves in the process area, in particular in the environment of the projection system resulting in excitation of the projection system or the other parts. Also, the air flows and/or pressure waves resulting from these movements may result in imaging errors such as overlay and/or focus errors.